# So You Think Your Critical HVAC System Is Reliable?

Thursday, May 02, 2013

I just read an article in Engineered Systems magazine that reminded me of one of my own blog postings from a few months ago.  The difference is that the author in the ES magazine article went all mathematical on us and showed the formulas for calculating HVAC system reliability given the reliability of the individual components in the system.  Although it was a long time ago I remember going through the mathematical exercise in one of my engineering courses back at the University of Texas...so we have all known about this procedure for a very long time.

During this same week I have been asked to do a competitive analysis on a "new" system concept compared to one of our systems.  While I could name names that is not necessarily the important point of this posting.  What struck me about the competitor's "new" system concept was just how many parts were required to accomplish the task of providing "free cooling".  Many of those component parts had dependencies that meant that the proper reliability analysis was the "series" analysis.  You can refer to the latest issue of Engineered Systems if you don't remember what that means but in keeping with my simple approach to my postings the bottom line is that the reliability of a "series" of components is the compounded product of each item's reliability multiplied together.  In other words if the "new" system concept required 4 compressors that are staged in series, a direct drive exhaust fan array, a direct drive supply fan array, a sensible heat wheel motor, sensible heat wheel belt, sensible heat wheel bearings, digital control module, etc...and we gave each of those items a reliability of 98% (which sounds pretty good and is generous for some of the items in the chain)...then the "system" reliability would be:

.98 x .98 x .98 x .98 (compressor section) x .98 (exhaust fan) x .98 (supply fan) x .98 (sensible heat wheel motor) x .98 (sensible heat wheel belt) x .98 (sensible heat wheel bearings) x .98 (digital control module) = .817

So, the "new" system concept under this scenario would actually have a reliability of only 81.7%...not quite so good I think you would agree.

The information from this example is actually directly from the competitor's product literature...and I left out some components for simplicity sake.  The reliability of the components at .98 was an estimate and you can plug in whatever numbers you think are accurate.  The important thing is to recognize that the more complex the system is the lower the reliability will be.

In a previous blog posting I quoted Albert Einstein who said ..."make everything as simple as possible, but not simpler.".  I still think Albert was a pretty smart guy and when I look at some of the design solutions being proposed today for data centers, pharmaceutical warehouses, or cooling in general I just have to wonder why we sometimes design such complex solutions.

Remember..."it is not sustainable if it is not maintainable"...and, as a corollary to that statement, "it is not maintainable if it is too complex and has too many parts"…